It has been possible to prepare a low-resistivity n-type ZnO by a conventional B, Al, Ga or In doping technique without any difficulty. However, as for p-type ZnO, there have been only reports on a high-resistivity p-type ZnO obtained by N (Nitrogen) doping. For example, one N-doped p-type ZnO was reported from Kasuga Laboratory of Engineering Department of Yamanashi University in the 59th Meeting of the Japan Society of Applied Physics (Lecture No. 17p-YM-8, Japanese Journal of Applied Physics, Part 2, vol. 36, No. 11A, p. 1453, 1 Nov. 1997). The p-type ZnO thin film prepared by Kasuga Laboratory of Engineering Department of Yamanashi University is not suitable for practical use because of still high resistivity of 100 Ω·cm. Further, the p-type ZnO thin film has a remaining problem of experimental repeatability in that its conduction type is inversely changed from p-type to n-type after annealing, and has not been developed to an effective low-resistivity p-type ZnO.
Li is one element of the first group of the Periodic System and is assumed as an acceptor for fabricating p-type materials. Heretofore, Li-doping has been used only for fabricating high-resistivity ZnO thin films, and such doping effects are being studied in a field of dielectric materials as electrical insulators rather than materials for semiconductor devices. For example, in the journal of the Physical Society of Japan, vol. 53, No. 4, pp. 282-286, Akira Onodera (Graduate School of Science, Hokkaido University) has reported to prepare a Li-doped ZnO having a high resistivity (specific resistance) of 1010 Ω·cm as a memory material by one crystal growth method, so-called hydrothermal method.
(Problem to be solved by the invention)
Out of ZnO having a p-type conductivity, various high-resistivity p-type ZnO thin films can be fabricated, whereas it has been difficult to grow a ZnO single-crystal thin film having a low resistivity with keeping the p-type conductivity due to self-compensation-effects and low solubility of p-type dopants.
If a p-type ZnO having a low resistivity can be synthesized as a ZnO single-crystal thin film, it becomes possible to achieve a p-n junction between ZnO (zinc oxide) semiconducting compounds of the same kind by combining the synthesized low-resistivity p-type ZnO with the low-resistivity n-type ZnO (zinc oxide) which has already been put into practice through the impurity doping process using B (Boron), Al (Aluminum), Ga (Gallium) or In (Indium). This p-n junction, referred to as a homojunction, makes it possible to fabricate various semiconductor devices, such as an implantation type light-emitting diode, laser diode and thin film solar cell, with high quality and low cost. For example, the above ZnO can be used for fabricating an ultraviolet laser diode necessary for high-density recording or large-scale information transfer.